Since the front door is locked,why not _____at the back door

i can't open the front door.Why not try ___the back door?A o －－I can't open the front door.----Why not try ___the back do This is a key ____ the front door.Why you try it at the back If no one answer the door,why not try knocking the back door He locked the door,______. they kept the door locked,door与locked的关系 Why is the door closed? We need ( )to open the locked door after breakfast ,Mr .Smith locked the door , She is in front of the door.(特殊划线部分是in front of the door) There seems to be no one at home -Why not()at the back door the ____ is not for the door.